Heating, ventilating and air conditioning systems (HVAC systems) make extensive use of air flow control devices, such as fixed and variable air volume (VAV) air diffusers, VAV boxes and return air flow control devices. HVAC systems typically include a network of ducts that extend from a central air supply or air source. These ducts extend to a plurality of rooms or spaces into which the supply air is to be discharged and typically at least one return air duct extends back from one of the spaces to the air supply source. Mounted on the various branches of the HVAC supply air duct network will be air diffusers which discharge the supply air into the room and which are used to control, actively or passively, the amount of air discharged. In a VAV diffuser there is a movable damper assembly which modulates the flow rate of air in order to control temperature or ventilation in the room. In a fixed air diffuser, there also will be a damper assembly that can be adjusted manually, but once adjusted, does not modulate or vary the air volume being discharged. In a fixed volume diffuser systems the air temperature usually is modulated at the source of the supply air to control room air temperature, or an “on” and “off” thermostatic control of air volume is used. Return air ducts also can be provided with adjustable damper assemblies to control the flow rate of air returned to the source.
When designing an air conditioning system using cooled air, the peak cooling load for each space is determined and from that load, the design air volume to be discharged into the space is calculated. Design air volume may vary between spaces as each space may have a different peak cooling load. For example, corner rooms. Both usually have a higher cooling load than interior spaces. The cooling load for each space may also include office equipment with large cooling requirements, such as a copy machine, if it is known where they will be located. The sum of various peak loads for a space determines the design air volume or flow for that space.
The purpose of air balancing an HVAC system is to achieve design air flow into each space at peak or design system air flow. Less than design air flow into a given space usually results in insufficient cooling, and more than design air flow results in overcooling. Air balancing insures that the air flow will be correct at peak or design cooling load conditions.
At times other than peak loading, cooling must be reduced. Cooling reduction for constant volume systems is accomplished by raising the supply air temperature and for variable air volume (VAV) systems by reducing the supply air volume.
When cooling is reduced, the system usually becomes unbalanced primarily because the cooling loads for each space do not reduce exactly in proportion to the design loads for each space, but instead, reduce in a random fashion. The remedy for this is to provide some form of control for each space. Constant volume systems may have a heating coil to raise the supply air temperature for that space (constant volume reheat). VAV systems may separately vary the air volume to each space, and the VAV diffuser is one method of varying the air volume to each space. Another more expensive approach to varying the air volume to each space is to provide a VAV box in the supply air duct for each space.
Method of HVAC System Air Balancing
Constant volume air systems, as a minimum, are comprised of an air handling unit which contains the fan, duct work to channel the air, and finally fixed constant volume diffusers to diffuse the air into the space. A balancing damper is located in each duct supplying each diffuser. The function of the balancing damper is to add pressure drop, both to compensate for the varying duct lengths to each diffuser and to obtain the correct static pressure at each diffuser for the specific design air volume of that diffuser. Also necessary is a channel or duct to return air from the space to the fan. Other devices may be included, such as reheat coils, and a method of introducing outside air into the supply air to meet ventilation requirements.
Balancing a constant volume system is accomplished by measuring the total air flow at the fan and the air flows through each diffuser and then adjusting and readjusting the fan speed and the balancing dampers for each diffuser until the air flows, both at the fan and through each diffuser, are all at design.
VAV box systems differ from constant volume in that VAV boxes are located between the fan and the fixed diffusers. When balancing, the boxes must be temporarily fixed wide open. Balancing is then done exactly the same as for a constant volume system, with duct-mounted balancing dampers, to achieve design air flow at both the fan and through each diffuser.
VAV diffuser systems differ from a constant volume system in that a VAV diffuser is used to control the supply air volume discharged instead of controlling the supply air temperature discharged from a fixed diffuser. These systems also may have static pressure controls such as zone dampers in the cutwork. When balancing a VAV-based system, both the VAV diffusers and the static pressure controls must be temporarily fixed open. Then, like VAV boxes, balancing is done using balancing dampers exactly the same as for a constant volume system to achieve design air flow at the fan and through each diffuser.
Part of the balancing process, whether the system is constant volume, VAV box or VAV diffuser, is to measure the air flow through each diffuser. Present methods for doing this are: 1) to use a “flow hood” to directly measure the flow through the diffuser, or 2) to measure the air velocity at the diffuser outlet and then use a factor (Ak) provided by the manufacturer to convert the velocity to air flow.
The present invention measures the pressure differential or drop over the diffuser by measuring the pressure in a known cross sectional area of the diffuser, such as the inlet of the diffuser, as compared to the space or room pressure. Pressure drop over a known area directly corresponds to flow through it. Therefore. a table can be provided to convert the pressure drop over the diffuser into an air flow rate.
Prior art “flow hoods” used to directly measure air flow through an air diffuser will typically include a hood portion which can be positioned over the outside of the diffuser so as to catch the air discharged from the diffuser. The hood is formed as a funnel which directs the discharged air down to a flow meter, such as an analog, dial-type, pressure differential measuring device, which is capable of measuring the discharge air flow rate.
Typical of prior art electronic flow hoods are the flow hoods manufactured by Dwyer Instruments, Inc. of Michigan City, Ind. and Short Ridge Instruments, Inc. of Scottsdale, Ariz.
The use of electronic flow hoods to balance HVAC systems is effective, but it also has been accompanied by numerous problems. Flow hoods are very bulky since they must extend around the diffuser discharge opening in order to capture substantially all of the air discharged from the diffuser. Flow hoods also have substantial height dimensions in order to funnel the air down to the meter. Thus, the balancing process is quite awkward and tedious. Still further, balancing some diffusers using a flow hood can essentially be impossible because of structures in the room or the configuration of the room itself. Thus, rooms with room-dividing partitions often interfere with placement of a flow hood around the diffuser. Similarly, alcoves, bookcases, furniture and the like make periodic checking of the diffuser balance extremely difficult or impossible. As a practical matter, HVAC systems often have diffusers which are only approximately balanced by air conditioning technicians, usually by working off of actual measurements of nearby diffusers.
Additionally, HVAC diffusers often have discharge openings which require custom flow hoods to catch the discharged air. While such custom hoods are available, they require the testing company to maintain an inventory of hoods so that they can do balancing on a variety of diffuser systems.
HVAC diffuser systems have previously included pressure sensing devices which are incorporated into the automatic controls regulating the volume of air discharged from the diffuser. Most of these systems include a pressure sensor that senses static pressure, or velocity pressure, or differential pressure, or various combinations thereof. In such systems the transducer is built into the HVAC system and a meter is provided at the pressure transducer assembly or at the control box for the overall system. Again, such built-in pressure sensing assemblies are effective and can be used to control the volume of discharge by calibrating the sensed pressure to the volume of flow discharged through the diffuser. This approach, however, requires that a dedicated flow meter be built into, or coupled to, each diffuser, and it accordingly increases the overall system cost substantially. Such a dedicated metering system clearly is not cost effective for, for example, relatively low-cost fixed-volume diffuser systems. Building in pressure meters for each diffuser in an HVAC system, therefore, provides more capability than necessary, particularly when considering the fact that balancing of such systems is only required at installation and only periodically thereafter.
U.S. Pat. Nos. 5,705,743; 5,364,304; 5,271,558; 4,591,093; 4,133,212 and 2,838,932 are typical of built-in pressure measuring systems and of anemometers that can be adapted for use in such systems.